The invention relates in general to bandpass filters. More particularly, it relates to radio frequency (RF) bandpass filters within a multilayered low temperature co-fired ceramic (LTCC) substrate.
As integrated circuits (IC) technology advances, more devices and modules are integrated into single chips to provide system-on-chip (SOC) and system-in-package (SIP) feature. As a result, telecommunication system has been demanding SOCs or SIPs on compact portable communication devices. Telecommunication systems also often have RF modules that require passive devices, such as resistors, inductors and capacitors. Devices such as these present difficulty in size reduction while maintaining desired performance and function. Passive devices increase area or volume occupation as SOC or SIP size decrease. Therefore, it is important to reduce passive device size or/and integrate them into the circuit board where SOCs or SIPs are mounted.
For example, a bandpass filter (BPF) and a balance/unbalance transformer (Balun) may be needed in an RF front end. RF circuit design has incorporated most desired functions into one or more chips, but BPF and Balun are among the exceptions. A conventional method of providing BPF and Balun in a telecommunication system individually mounts them on the surface of a circuit board. Since BPF and Balun are among the thickest in comparison with other individual devices, size reduction of total system volume can be difficult.
One technique, seeing increased use in dealing with this difficulty, is use of multilayered LTCC substrates, wherein ICs and other chip components are mounted on the to surface, while passive devices are formed among the underlying layers. A traditional BPF in a multilayered LTCC substrate has coupled stripline elements to interact with an external, individual parallel-plate capacitor. To provide required coupling capacitance, the external parallel-plate capacitor cannot be overly thin, such that and the total system size is excessive.